In recent years, conventional solvent type coatings are being shifted to aqueous coatings in order to prevent the air pollution caused by the organic vapors emitted from said solvent type coatings.
Ordinary aqueous coatings for can comprise a film-forming resin and a crosslinking agent. As the crosslinking agent, there is mainly used an alkyl-etherified melamine resin or an alkyl-etherified benzoguanamine resin.
As these amino resins, there is used an alkyl-etherified amino resin containing a functional group such as imino group, methylol group or the like, or a completely alkyl-etherified amino resin. There is also used an amino resin containing a mononuclear component in a large amount, in order to allow the resulting aqueous coating to have sufficient water solubility.
When such an amino resin is used as the crosslinking agent, however, a vapor of low-molecular weight substance is emitted from the alkyl-etherified amino resin during the baking of the aqueous coating composition containing the amino resin, and reaches the inner wall of the curing oven and condenses; the condensate drops on the coated article; resultantly, there occur problems such as the deterioration of coated surface, the necessity of frequent cleaning of oven inside, and the like. Hence, an improvement therefor is desired.
There also remain, in some cases, a problem of the insufficient stability of coating caused by an alkyl-etherified amino resin of insufficient water solubility, and a problem of poor resistance to pasteurization, insufficient hardness and processability, caused by the high content of mononuclear component.
The techniques for making water-soluble a water-insoluble alkyl-etherified amino resin are disclosed in U.S. Pat. No. 3,464,946, U.S. Pat. No. 3,444,114, etc. When the resulting amino resin is used as a crosslinking agent for outer surface coating of can, however, the coating is not satisfactory in fuming during baking as well as in film properties such as resistance to pasteurization, hardness, processability and the like.